Hello,
It seems like most alt coins are trading against the bitcoin, people enter orders in bitcoins to buy alt coins.
What happen when the price of bitcoins go up ? The orders for the alt coins still the same, the market value of the alt coins is inflated. If bitcoin go up, then all the alt coins will go up, and if bitcoin go down, then they will also all go down.
If bitcoin goes up 10 fold most alt coins will also go up 10 fold. It's a pyramid.
The global market cap seems to be higher and higher because of this simple fact, but the real money it's only inside of bitcoin.